Have you tried asking Mr Google "Where can I buy British fish?"
I just did & was pleasantly surprised to find a vendor offering a lovely selection of British fish caught in West Country waters. Looks fantastic. If I had a bigger freezer I'd order a selection box myself.
I hope this is helpful info.
Estranged Son and Future Granddaughter
To think that London, or anywhere else for that matter, does not belong to any one demographic